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            <div><h4 class="title">
                <a name="asio.overview.cpp2011.futures"></a><a class="link" href="futures.html"
                                                               title="Futures">Futures</a>
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    <p>
        The <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">use_future</span></code> special value provides first-class
        support for returning a C++11 <code class="computeroutput"><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">future</span></code>
        from an asynchronous operation's initiating function.
    </p>
    <p>
        To use <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">use_future</span></code>, pass it to an asynchronous
        operation instead of a normal completion handler. For example:
    </p>
    <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span
            class="identifier">future</span><span class="special">&lt;</span><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span> <span
            class="identifier">length</span> <span class="special">=</span>
  <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span
                class="special">(</span><span class="identifier">my_buffer</span><span class="special">,</span> <span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">use_future</span><span class="special">);</span>
</pre>
    <p>
        Where a handler signature has the form:
    </p>
    <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span
            class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span
            class="identifier">result_type</span> <span class="identifier">result</span><span class="special">);</span>
</pre>
    <p>
        the initiating function returns a <code class="computeroutput"><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">future</span></code>
        templated on <code class="computeroutput"><span class="identifier">result_type</span></code>.
        In the above example, this is <code class="computeroutput"><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">size_t</span></code>.
        If the asynchronous operation fails, the <code class="computeroutput"><span class="identifier">error_code</span></code>
        is converted into a <code class="computeroutput"><span class="identifier">system_error</span></code>
        exception and passed back to the caller through the future.
    </p>
    <p>
        Where a handler signature has the form:
    </p>
    <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span
            class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
    <p>
        the initiating function returns <code class="computeroutput"><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">&gt;</span></code>. As above, an error is passed back
        in the future as a <code class="computeroutput"><span class="identifier">system_error</span></code>
        exception.
    </p>
    <p>
        <a class="link" href="../../reference/use_future.html" title="use_future">use_future</a>, <a class="link"
                                                                                                     href="../../reference/use_future_t.html"
                                                                                                     title="use_future_t">use_future_t</a>,
        <a class="link" href="../../examples/cpp11_examples.html#asio.examples.cpp11_examples.futures">Futures example
            (C++11)</a>.
    </p>
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